There has been known a technology in which an electrically heated catalyst is arranged at a location inside of an inner pipe of a double pipe (for example, see a first patent document).
In addition, there has also been known a technique in which a catalyst is arranged between an inner pipe and an outer pipe of a double pipe, wherein the inside diameter of a portion of the inner pipe at the upstream side of the catalyst is made smaller as it goes toward the upstream side (for example, refer to a second patent document).
Moreover, there has also been known a technique in which a catalyst is arranged at a location between an inner pipe and an outer pipe of a double pipe, or at a location inside of the inner pipe, wherein the inside diameter of a portion of the inner pipe at the upstream side of the catalyst is made smaller as it goes toward the upstream side (for example, refer to a third patent document).
Further, there has also been known a technique in which a catalyst is arranged at a location between an inner pipe and an outer pipe of a double pipe, and another catalyst is arranged at a location inside of the inner pipe, wherein the inside diameter of a portion of the inner pipe at the upstream side of the catalyst is made smaller as it goes toward the upstream side (for example, refer to a fourth patent document).
However, when particulate matter (PM) in the exhaust gas adheres to an inner pipe of a double pipe, there will be a fear that upon electrical energization of an electrically heated catalyst, electric current may pass to a case through the particulate matter. In order to suppress this, for example, it is possible to adopt a structure (a labyrinth structure) which serves to make it difficult for particulate matter to come or flow into between the inner pipe and the case. However, it is difficult to prevent the inflow of particulate matter in a complete manner. Here, the particulate matter having adhered to between the inner pipe and the case can be oxidized and removed by raising the temperature of the inner pipe. However, with the above structure, it will become difficult for the exhaust gas to flow through between the inner pipe and the case, the extent of the rise in the temperature due to the heat of the exhaust gas becomes small. For this reason, it becomes difficult to remove the particulate matter having adhered to between the inner pipe and the case. As a result, there is a fear that electricity may flow to the case through the particulate matter.